The present invention relates to a method for the preparation of a dehydrated quartz glass material for light transmission or, more particularly, to a method for the preparation of a dehydrated quartz glass material for light transmission containing substantially no residual chlorine as an impurity which increases the transmission loss of light through the quartz glass.
As is known, optical fibers used for the communication technology by light transmission are usually manufactured by spinning a quartz glass material which is produced by the deposition of fine silica particles formed by the flame hydrolysis of a hydrolyzable or oxidatively decomposable silicon compound, e.g. silicon tetrachloride, in a high-temperature flame, e.g. oxyhydrogen flame, on a substrate body to be grown in a porous cylindrical silica body which is then vitrified by heating at a temperature around or above the melting point of silica in a high-temperature furnace into a transparent fused quartz glass body.
One of the problems in the above described method for the preparation of a quartz glass body for light transmission is that a considerably large amount of hydroxy groups is contained in the fused quartz glass body as a natural consequence of the moisture content in the oxyhydrogen flame so that the optical fiber obtained from such a quartz glass body as such by spinning is not suitable for the purpose of light transmission due to the decreased efficiency or increased loss in the light transmission by the impurity hydroxy groups.
As a remedy for this drawback, it is a usual practice that the hydroxy groups are dehydrated and removed from the quartz glass material by exposing the porous silica body before vitrification in a high temperature atmosphere containing chlorine or a chlorine compound such as thionyl chloride. This method is also not free from problems and disadvantages. Although the transmission loss by the hydroxy impurity can be reduced, the quartz glass material unavoidably contains chlorine as another impurity which causes problems in a different way. Namely, the chlorine impurity in a quartz glass material is not responsible for the light absorption in a wave length region of 0.6 to 1.5 .mu.m frequently used in the communication by light transmission but, as is understood from the comparison of the bond energy between silicon and chlorine atoms and that between silicon and oxygen atoms, the former value being definitely smaller than the latter value, the silicon-to-chlorine linkage is subject to scission or breaking during a long-term service of the optical fiber or when the optical fiber is heated at an elevated temperature to form structural defects responsible for the increase of the light transmission loss. This problem of increased light transmission loss by the chlorine impurity is more serious when the optical fiber is used in an atmosphere containing a relatively high concentration of hydrogen gas or moisture. In short, optical fibers prepared from a chlorine-containing quartz glass material are not satisfactory in respect of the long-term stability and reliability.
In order to solve this problem advantageously, it has been proposed to treat the porous silica body with fluorine gas or a gaseous fluorine compound, such as fluorinated or fluorochlorinated hydrocarbons, e.g. tetrafluoromethane and difluorodichloromethane, in place of chlorine or a chlorine compound to introduce silicon-to-fluorine linkages having a larger bond energy than the silicon-to-oxygen linkage into the quartz glass. The use of fluorine gas in the above purpose is, however, not practicable because the excessively high reactivity of fluorine with siliceous materials causes erosion and dissipation of the silica material from the porous body in the form of silicon tetrafluoride. The above mentioned fluorine compounds, on the other hand, are chemically extremely stable so that the desired effect of dehydration can be obtained only by the use thereof in an excessively large amount while loss of the silica material in the form of silicon tetrafluoride is also unavoidable by the use of such a large amount of the fluorine compound. Moreover, the carbon content in the fluorine compound acts as a reducing agent on the silica SiO.sub.2 into more volatile silicon monoxide SiO to increase the amount of silica dissipation and also is responsible for the increased carbon content and formation of oxygen deficiency in the quartz glass material.